Add a `util/` script for installing basic system-wide tools

The script should automatically use either `brew` or `sudo apt-get` for installation as appropriate.

To install:

- macOS: `brew` (if not already present) ?
- `rg`
- `jq`
- `python3`
    - It might be easier to just assume this is always already installed.
- `wget`
- `tmux`
- Debian:
    - `python3-dev`
    - `python3-venv`
    - `build-essential`?

Cf. `util/cloud-config.yaml`